Investigators are looking into whether the firefighting response to the Deepwater Horizon rig explosion contributed to its sinking, the Coast Guard says.

Word of that line of inquiry came in a report issued Wednesday by the Center for Public Integrity. Coast Guard spokesman Capt. Ron LaBrec told the watchdog group that "the joint investigation is absolutely looking into that, and whether it contributed to the sinking."

The Coast Guard confirmed LaBrec's statement in an e-mail to msnbc.com.

The concern focuses on the fact that responders used saltwater on the burning rig instead of retardant foam. That could have overwhelmed the ballast chambers that kept the rig afloat with air and water, changing its weight distribution and causing it to list and then sink.

The wellhead on the seafloor was already leaking oil before the rig sank two days after the April 20 explosion, but it's possible the sinking could have made it harder to attack the leak since the piping attached to the well ruptured with the sinking.

The Coast Guard has said none of its ships was equipped to fight a rig fire and that they focused instead on search and rescue.

"We have expertise in fighting a fire on board our vessels, but since firefighting is not one of our missions, we do not train for rig fires and that has really been the responsibility of the rig owner and operator," LaBrec said.

Instead, the firefighting effort was led by a half-dozen private boats, the center said.

LaBrec acknowledged that spraying salt water onto a burning rig may have affected the ballast. "In the end, it may really depend on what agent is available and in this case it appears it was salt water only," he said.

Experts interviewed by the center agreed that salt water can affect a rig's balance, but they disagreed on whether it would have mattered in Deepwater Horizon's case since the explosion was so severe.

The center also cited testimony before the Deepwater Horizon investigative commission suggesting that the Coast Guard did not follow its own procedure to put someone in charge of firefighting in such a disaster.

Kevin Robb, a civilian Coast Guard specialist who was the first watch commander the night of the accident, said at a May 11 hearing that he knew of no attempt to designate a fire marshal.

The center cited testimony by the Coast Guard official chairing the investigation, Capt. Hung Nguyen, who then questioned whether that chaotic response contributed to the sinking.

"So what we're looking at here is maybe if there's no coordination out there, no direction out there, we maybe throwing water onto a disabled vessel that may lead to this sinking; is that correct? Is that the potential?" Nguyen asked Robb.